The invention relates to directional control valves, and particularly to such valves that are pilot-operated and incorporate fail-safe features.
Characteristically, shear seal type valves afford the advantage of low operating force. Typically, non-interflow shear seal valve arrangements are limited in flow capacity because the internal valving flow paths are held to limited dimensions so that the host valving elements are not overly large and the valve housing body is of a practical cost-effective size. Restricted flow capacity in a valve can adversely affect response time of the control circuit in which it is employed.
Other factors which can seriously delay response of a pilot-operated valve are long pilot control lines and compressibility of the fluid in the line. Blow-off preventer systems in underwater oil and gas wells are one application where a relatively long pilot control line may be required. By way of example, a pilot control line of a length of 27,000 feet, for example, and using a water-based control fluid, may exhibit a delay of 11/2 minutes in a pressure signal change from 220 psi to 600 psi.